1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine which impresses a high voltage, for generation of spark discharge, on a spark plug for each engine cylinder.
2. Description of the Related Art
As a conventional ignition apparatus for an internal combustion engine, there has been known one in which a conductive layer is formed on an inner surface of a case made of an insulating plastic that receives therein a transformer comprising a primary coil, a secondary coil and an iron core, and a high voltage, which is generated at a high voltage side of the secondary coil upon interruption of a primary current supplied to the primary coil, is impressed to a spark plug so as to generate a spark discharge in a gap portion between electrodes (see, for example, a first patent document: Japanese patent application laid-open No. 2004-241621).
In this ignition apparatus for an internal combustion engine, noise contained in a discharge current generated by the discharge in the gap portion is radiated into the air from the transformer through the conductive layer formed on the inner surface of the case in the air, whereby electrical devices mounted on a vehicle are prevented from malfunctioning.
In the above-mentioned known ignition apparatus for an internal combustion engine, there is a problem that if the shape of the case is complex, a considerable time and effort is required, when the conductive layer is formed on the inner surface of the case, due to the difficulty in the formation of the conductive layer on the case inner surface.
In addition, the conductive layer is formed by adhering or bonding a thin metal sheet onto the inner surface of the plastic case, and in such a case, there arises another problem that the conductive layer might be cracked by the expansion and contraction operation of the conductive layer due to temperature changes, as a result of which a discharge might be generated in the interior of the cracked conductive layer by a high voltage of the secondary coil, thus generating noise accompanying the discharge.